I bought Neil Stanton's S-CAB system in late-2012. At the time it was only sold through North West ShortLine (NWSL). However, in 2013 Neil created the S-CAB web site and started to sell the products directly. Many of the systems mentioned on the "Battery-powered Systems" page were not yet available at the time I made my purchase. However, even today, looking at what is available, I still find that the S-CAB system is the most complete package with a minimum of fuss.
The photo shows the S-CAB throttle (the previous version), and an S-Helper Service NW2, which was my first battery-powered engine that I converted. The engine's number is 5912, so the throttle is set to "12", and I am running it on my layout.
(external link: S-CAB)
Fundamentally, S-CAB is a radio-frequency train control system. The throttle sends your commands to the receiver in the locomotive. This removes the train-control signals from the rail, thereby making communication easier and better. Secondly, once Neil built that system, he continued and incorporated the ability to power all of the electronics within a locomotive from a rechargeable cell-phone-sized battery. The battery-power system is separate from the core S-CAB system. However, I bought both the S-CAB and the battery-power components, since the latter was really what I was after. The beauty of Neil's S-CAB system is that it is a completely self-contained system. It includes components to safely charge the battery from any electricity detected on the rails, and it safely shuts down the system when the battery has reached its drained state. This allows you to let the engine charge via a piece of powered track, and let it sit overnight without any worries.
The S-CAB throttle sends your commands to the receiver circuit board installed in your locomotive. That board then converts the commands into the standard DCC commands that the DCC decoder installed in your locomotive can understand. The DCC decoder then interacts with the various parts in your locomotive to execute the command that you gave it (turn on the lights, move forward, go faster, etc.). This communication between the throttle and the receiver board is done via radio-frequency, so you do not need to aim the throttle at the locomotive.
If you have the battery-power components installed, the electricity comes from the battery (left in the photo). The decoder is powered by the battery installed in the locomotive. The decoder then sends that electricity to the motor, LEDs, and speakers (if so equipment) to power those parts. The decoder is always powered from the battery, even if your locomotive is running on track that has power applied to it (this is slightly different in the latest versions of the S-CAB system).
You can run on track that has no power (i.e. "dead-rail"), or that has AC, DC, or DCC applied to it. This means that you can take your engine to your friend's or club's layout and run it, regardless of what system they are using. Inside the locomotive is an extra circuit board called the Battery Power Supply (BPS; shown on the right in the photo) that is in control of charging the battery, shutting the battery power down, and upconverting the voltage of the typical cell-phone batteries (most supply around 3.7 volts, but our locomotives need around 12 volts). This extra board does add one more thing that you have to fit within your locomotive, but the peace of mind that it provides is well worth it.
Combined with a small circuit board that is attached to the top edge of the battery, the BPS prevents the battery from being overcharged (which would cause the battery to explode), or to drain too much (which would prevent the battery from ever being able to hold a charge again). It also manages the various states of charging and not charging the battery depending on whether or not your locomotive is running on dead rail and is still equipped with power pick-up connections.
So, with the radio-frequency control and with the battery for power, a locomotive is able to run on unpowered track. All you need are the components inside the locomotive and the throttle. No other components are needed.
By the way, in 2017 Neil released a new version of the BPS board that allows it to pick-up power from the rail, if the battery is discharged, to continue to power the DCC decoder in the engine. This is great for train shows!
(external link: Testing Li-Po Batteries)